IMP: allow user to supply backbone tree for RAxML

[mikerobeson]

Improvement Description
To decrease the time it takes to construct a phylogeny, it'd be helpful to allow the user to provide a constaint or backbone tree via the -r flag, as outlined in this RAxML Tutorial. Basically, this enforces a topological constraint on the resulting output tree. That is, it ignores paths in tree space that violate the constraint topology. IQ-TREE can also do this.

References

1. RAxML Tutorial
2. The initial feature request is here.